The present invention relates to a method of cutting a wood block by a veneer knife for peeling veneer therefrom and also to a veneer lathe.
Wood veneer used for manufacturing various wood products such as plywood and laminated veneer lumber (LVL) is produced by cutting or peeling a generally cylindrical wood block by a veneer lathe. As is well known in the art, the peeling is accomplished by moving the cutting edge of a veneer knife into the rotating wood block at a controlled feed rate. For improved veneer yield, it is important in veneer peeling operation that the wood block should be reduced to a core stock with as small diameter as possible while ensuring the strength of the wood block to resist external force applied thereto during the peeling operation. A veneer lathe is disclosed in U.S. Pat. No. 5,141,038 assigned to the same present assignee which is capable of cutting a wood block to a small diameter while preventing the wood block from being flexed or bent during the veneer peeling operation.
FIG. 28 is a schematic side view showing the above-identified veneer lathe. In the drawing, numeral 201 designates a knife carriage of the veneer lathe which is equipped with a veneer knife 205 having a cutting edge cutting into a wood block 207. The wood block is prepared by cross-cutting a log into the desired length. The knife carriage 201 has a pair of leadscrews 203 (only one being shown) each engaged with a leadscrew nut (not shown) having formed through an internal thread and connected to a servomotor 204 for driving its leadscrew 203 to rotate. The servomotor 203 is operable to rotate the leadscrew 203 reversibly in response to a signal from a control unit (not shown), thereby moving the knife carriage 201 toward and away from the wood block 207 as indicated by double-headed arrow. An absolute rotary encoder (not shown) is connected to the servomotor 204 for measuring or determining the current position of any part of the movable knife carriage 201, e.g. the cutting edge of the veneer knife 205 mounted on the knife carriage 201, with respect to a predetermined reference position of the veneer lathe. The use of this absolute rotary encoder makes possible determination of the distance r between the cutting edge of the veneer knife 205 and the spin axis of the wood block 207 that coincides with the axis of rotation of spindles when a wood block is supported by such spindles.
Numeral 209 designates veneer peeled from the wood block 207 by the veneer knife 205. Numeral 211 designates a number of drive discs (only one disc being shown) disposed adjacently to the cutting edge of the veneer knife 205 and juxtaposed in parallel relation to the spin axis of the wood block 207 for driving the wood block 207 from the periphery thereof. Each disc has formed on the outer periphery thereof a number of piercing projections engageable with the periphery of the wood block 207. The drive discs 211 are drive to rotate at a constant speed by a servomotor 213 via chains 215 in the direction that causes the wood block 207 to rotate in arrow direction through the engagement between the wood block 207 and the discs 211. Numeral 217 designates a plurality of nose bars (only one nose bar being shown) so as to press the periphery of the wood block 217 at a position immediately above the cutting edge of the veneer knife 205 for reducing lathe checks appearing in the surface of peeled veneer 209.
Numeral 219 designates a first roll disposed on the opposite side of the wood block 207 from the cutting edge of the veneer knife 205, freely rotatably supported by a holder (not shown) by way of a bearing (not shown either) and having such an axial length that the first roll 219 is contactable with the periphery of the wood block 207 over the entire length thereof. The holder for the first roll 219 is engaged with a leadscrew 221 driven to rotate by a servomotor 220 operable in response to a signal generated by the control unit (not shown) thereby to move the first roll 219 reversibly in horizontal directions as indicated by double-headed arrow. An absolute rotary encoder (not shown) is connected to the servomotor 220 for measuring the distance between the spin axis of the wood block 207 and the point at which the first roll 219 is in contact with the periphery of the wood block 207 during veneer peeling operation.
Numeral 223 designates a second roll freely rotatably supported by a holder (not shown) by way of a bearing (not shown either) and having substantially the same axial length as the first roll 219 so that the second roll 223 is contactable with the periphery of the wood block 207 over the entire length thereof. The second roll 223 has a diameter that is smaller than that of the first roll 219 and idle so that the roll 223 is driven to rotate by the contact with the wood block 207 being rotated. The holder for the second roll 223 is connected to a leadscrew 225 driven to rotate by a servomotor 224 operable reversibly in response to a signal generated by the control unit thereby to move the holder and hence the second roll 223 in vertical directions as indicated by double-headed arrow. An absolute rotary encoder (not shown) is connected to the servomotor 224 for determining the distance between the spin axis of the wood block 207 and the point at which the second roll 223 is in contact with the periphery of the wood block 207 during veneer peeling operation. A rotary encoder (not shown) is connected to the second roll 223 for determining the peripheral speed of the second roll 223 and hence the peripheral speed of the wood block 207 that drives the rotary encoder in contact therewith. This rotary encoder is connected to the control unit and generates to the control unit signals indicative of such peripheral speed.
In peeling veneer from the wood block 207 in the veneer late of FIG. 28, the movement of the knife carriage 201, the first roll 219 and the second roll 223 is controlled as follows. The distance r between the spin axis of the wood block 207 and the cutting edge of the knife 205 is provided by the absolute rotary encoder connected to the servomotor 204 and the peripheral speed x of the wood block 207 is provided by the rotary encoder for the second roll 223. Based on the distance r and the peripheral speed x, the control unit figures out the number of revolutions per unit time n of the wood block 207. Since the driving discs 211 are driven to rotate at a constant speed, the wood block peripheral speed n is also substantially constant.
Based on the information of the block speed n, the control unit generates signals to the servomotors 204 so that the knife carriage 201 is moved for a predetermined distance for each complete revolution of the wood block 207. Value r representing the distance between the spin axis of the wood block 207 and the cutting edge of the knife 205 is reduced progressively while the knife carriage 201 is moved toward the spin axis of the wood block 207. Since the peripheral speed x of the block speed 207 is constant, the speed at which the knife carriage 201 is moved is increased with a decrease of the distance r.
The control unit is operable to control the operation of the servomotor 220 for the leadscrew 121 in such a way that the contact point of the first roll 219 with the periphery of the wood block 207 is positioned at a distance r from the spin axis of the wood block 207 and such contact point is maintained while the knife carriage 201 is being moved toward the spin axis of the wood block 207 and the diameter thereof is being reduced progressively, accordingly. That is, the controlling is made so that the first roll 219 is kept in contact with the periphery of the wood block 207 by being moved horizontally in accordance with the movement of the knife carriage 201 toward the spin axis of the wood block 207.
The control unit is also operable to control the operation of the servomotor 224 for the leadscrew 225. The controlling is made in such a way that the contact point of the second roll 225 with the periphery of the wood block 207 is positioned at a distance r from the spin axis of the wood block 207 and such contact point is maintained while the knife carriage 201 is being moved toward the spin axis of the wood block 207. This is accomplished by moving the second roll 225 vertically in accordance with the movement of the knife carriage 201 toward the spin axis of the wood block 207.
In such veneer lathe, a wood block can cut into veneer having the desired thickness without being bent or flexed by any external force produced during the wood block cutting operation and also the wood block can be peeled down to a smaller core diameter than in the case when the wood block is cut while being supported by spindles, which greatly contributes to improvement of veneer yield.
In such veneer lathe, however, the second roll 223 is brought into an interference contact with the veneer knife 205, as shown in FIG. 29, or the first roll 219 may be moved into a damaging contact with the second roll 219 when the block cutting has proceeded and the wood block 207 has been reduced to a certain core diameter. Obviously, no further cutting can be performed and, therefore, no further improvement of veneer yield is achieved.